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Both North American bladder snails Physella are much larger than the native Physa fontinalis. Physella gyrina is particularly large. The shell is fairly strong, deep horn-coloured and with shallow sutures and has a broader, blunter apex than P. acuta. The animal is dark grey and covered in striking whitish spots. Mostly in Northern Ireland - common on Lough Neagh - but being spread with fish hatchery products.

Key characteristics

A medium to large, sinistral, slightly glossy shell with a very large body whorl

Aperture wide and lip often reflexed outwards in shells from Lough Neagh

Animal mid to dark grey with striking whitish spots

Size

12-22 mm.

World Distribution

This naturalised North American species may have been introduced by the cotton trade into canals and similar habitats in Britain in the nineteenth century (Dean, 1920). It is not known with certainty in mainland Europe. It is native to central and northern parts of North America where it occupies shallow lakeshore habitats and temporary water-bodies (Jokinen, 1992).

Irish Distribution

Several brief investigations of the molluscan fauna of Lough Neagh in the 1970s and early 1980s, failed to detect this species (Anderson, 1977, 1983). An unusual physid was observed in an aquarium at the Lough Neagh Discovery Centre at Oxford Island, Armagh on 28 September 1995. Search of the immediate area revealed the species to be abundant in the open waters of the Lough. It was subsequently identified as Physella gyrina and found to be common throughout the Lough Neagh system and the Lower Bann as far as the tidal weir at The Cutts, near the north Antrim coast. Its provenance is unknown. A very isolated site was subsequently discovered in the Lough Eske catchment of south Donegal about 100 km west of Lough Neagh. Lough Eske is a small and isolated catchment draining directly into the Atlantic Ocean. Routes for accidental introduction are quite limited but it was likely transported with trout fry or parr in a re-stocking programme from The Cutts hatchery. It has since been found in the Ballynahinch (Quoile) River in Co. Down.

Ecology

This large and robust bladder snail is restricted to shallow inshore areas and marshes or to seasonally flooded shorelines in its Lough Neagh, Lough Eske and Quoile River localities

Lough Neagh is a hypertrophic water body already heavily populated by native molluscs but P. gyrina appears to have found a favourable niche and is abundant throughout

In summer it clusters abundantly under stones on shallow gravelly foreshores, almost to the exclusion of other molluscs but disappears in winter, presumably migrating into deeper water.